The functional significance of various methylated nucleosides present in the nucleic acids of tumor cells and of DNA- and RNA-tumor virsues are being investigated by various immunochemical techniques. This approach employs antibodies specific for various methylated nucleosides which include 7-methylguanosine (m7G),N2,N2-dimethylguanosine (m2G), N6-methyladenosine (m6A), and 5-methylcytidine (m5C). Upon production, characterization, and purification, antibodies will be employed to isolate and characterize nucleic acid possessing the corresponding methylated nucleosides. Nucleic acids of particular interest include Adenovirus DNA and mRNAs, Rous Sarcoma Virus RNA and various bateriophages such as fd. The interaction between a specific anti-methylated nucleoside antibody and various nucleic acids provides unique immunochemical approaches for the (i) detection and (ii) quantitation of methylated constituents present in nucleic acids. Additionally, electron micrographs of nucleic acid:antibody complexes can be utilized (iii) to map the position of methylated nucleosides within a nucleic acid while immobilized antibodies can be employed (iv) to isolate hapten-containing (deoxy)oligonucleotides and nucleic acids. Last, the inhibition of specific enzyme activities associated with the metabolism of nucleic acids by the appropriate nucleic acid:antibody complexes will provide information regarding the (v) functional role(s) of the methylated nucleosides being examined.